Environmental concerns have motivated the implementation of emission requirements for internal combustion engines and other combustion systems throughout much of the world. Catalytic converters have been used to eliminate many of the pollutants present in exhaust gas; however, an adsorption/catalyst medium is often used to remove nitrogen oxide (NOx) gases, such as, for example, NO and NO2, which are produced as a byproduct of the combustion process. Lean NOx traps (LNT), for example, are widely used in after-treatment systems for removing NOx from both gasoline direct injected engine (GDI) and light-duty diesel engine (LDD) exhaust gas. One LNT configuration comprises a NOx adsorber/catalyst (i.e., storage) material coated on a porous ceramic matrix with parallel passageways through which exhaust gas may flow, sometimes referred to as a honeycomb catalyst substrate or filter. NOx subsequently adsorbs onto the solid surface of the storage material during an engine's lean-burn mode, and is desorbed (i.e., reduced) during rich air-to-fuel mixtures. In other words, NOx can be stored during lean exhaust (e.g., high O2) conditions and released as N2 during rich (e.g., low O2) engine operation.
A NOx adsorber/catalyst generally stores NOx on a washcoat that is applied to a ceramic article, such as, for example, a porous substrate or filter. The washcoat blankets the walls defining the cells of the ceramic article with the storage material, providing a solid surface for NOx gas adsorption. Commercial washcoat blends, for example, may contain known NOx sorbents, which may include, by way of example only, platinum (Pt), rhodium (Rh), palladium (Pd), alkaline earth metals (e.g., magnesium (Mg), calcium (Ca), strontium (Sr) and barium (Ba)), alkali metals (e.g., lithium (Li), sodium (Na), potassium (K) and cesium (Cs)), rare earth elements (e.g., lanthanum (La) and yttrium (Y)), ceria, zirconia, titania, and compounds and compositions thereof. A washcoat's NOx adsorption ability is, therefore, generally determined based on the adsorbent performance of the sorbent used. For example, a potassium-based washcoat may be desirable because it may offer a relatively broad and high operating temperature window for NOx reduction activity, with less susceptibility to sulfur poisoning.
Some ceramic article materials, however, may react with potassium, thereby compromising the durability of the article and reducing the efficiency of NOx adsorption. Consequently, ceramic articles with high potassium-durability, such as, for example, ceramic articles based on blends of aluminum titanate and zirconium titanate (AT/ZT), have been developed. AT/ZT ceramic articles, for example, have demonstrated potassium-resistance during aging of the structure.
It may be desirable to provide ceramic articles that are relatively strong. Depending upon a particular application, it also may be desirable for ceramic articles to meet various performance requirements, for example, relating to strength, porosity and/or thermal expansion. For example, ceramic articles, including, for example, both filters and catalyst substrates, which are used as a base for a catalyst washcoat, generally require a relatively high total porosity to allow exhaust to pass through the walls of the structure. The ceramic article's strength, however, may be sacrificed with an increase in porosity. It may be desirable in at least some applications, therefore, to provide a ceramic article that is both porous and strong, and that exhibits potassium-durability and desirable thermal expansion properties. Accordingly, it may be desirable to increase the strength of a ceramic article, while substantially maintaining its porosity. It may further be desirable to increase the strength of a ceramic article, without negatively affecting its thermal expansion properties or its potassium-resistance.